The field of the invention relates to speed control systems for motor vehicles.
Prior speed control systems attempted to maintain vehicle speed at a reference speed by utilizing feedback control of the engine throttle in response to an error signal. Proportional feedback control is commonly used wherein the speed error signal is multiplied by a gain constant and the throttle moved in a direction to reduce the speed error signal towards zero. A problem with pure proportional feedback control is that the control system is continuously providing corrections, specially on hilly road surfaces, resulting in some speed oscillation which may be unappealing to the vehicle operator.
In an attempt to address the above problem, integral feedback control has been utilized wherein the error signal is integrated, or averaged, to provide the feedback control system. A disadvantage of such approaches is their inherently slow response to speed changes such as when encountering upgrades or downgrades in the road surface.
It is also known to provide proportional plus integral feedback control in an attempt to achieve both stable operation and rapid correction to alterations in road surfaces. Other systems have also added differential feedback control to correct for sudden changes in vehicle operation.
The inventors herein have recognized numerous disadvantages of the above approaches. More specifically, combining proportional, integral, and/or differential feedback control is a compromise solution which is not satisfactory under all road conditions. For example, the feedback system will still respond to minor variations in road surfaces resulting in a vehicle speed which appears to hunt. Another disadvantage occurs when traveling uphill. The rapid throttle change required for adequate acceleration may be absent in prior approaches. And, when cresting the hill, a rapid decrease in throttle position may be required to prevent speed overshoot which the operator may find unappealing.